1. Field of the Invention
This invention relates to a semiconductor light-emitting device used as a semiconductor laser.
2. Description of the Prior Art
As well known, wavelengths of semiconductor lasers widely used as light-sources for emitting coherent light depend on the materials that constitute the semiconductors used therein. Since, however, the oscillation wavelengths obtained have a descrete distribution throughout the infrared to visible regions, some of conventional semiconductor lasers have wavelength regions that have not been made actually attainable.
Detailed studies on the laser wavelengths having been attained by semiconductor lasers have revealed that semiconductor lasers having been put into practical use, in particular, semiconductor lasers comprised of an InP substrate and also an InGaAs material can be used in the bands of 1.3 .mu.m and 1.55 .mu.m; semiconductor lasers comprised of a GaAs substrate and also an AlGaAs material, in the band of 0.8 .mu.m; and semiconductor lasers comprised of a GaAs substrate and also an AlGaInP material, in the band of 0.6 .mu.m. In these semiconductor lasers, it is expected that changes made in the composition of a light-emitting portion, i.e., an active layer, while making a lattice match enable a change in an oscillation wavelength within the range of from 1.2 .mu.m to 1.6 .mu.m with regard to the InGaAs semiconductor lasers; within the range of from 0.78 .mu.m to 0.85 .mu.m with regard to the AlGaAs semiconductor lasers; and within the range of from 0.58 .mu.m to 0.67 .mu.m with regard to the AlGaInP semiconductor lasers, respectively. Material semiconductor lasers comprised of a GaSb substrate can effect oscillation in the wavelength region on the side longer than 1.6 .mu.m, and semiconductor lasers comprised of any of II to VI group materials, in the visible region. None of the material semiconductor lasers described above, however, can effect oscillation in the wavelength region of from 0.85 .mu.m to 1.1 .mu.m.
In further examples, AlGaAs semiconductor lasers can effect oscillation in the wavelength region of from 0.68 .mu.m to 0.78 .mu.m. The semiconductor lasers of this type, however, have no good device characteristics such that they have high threshold values and low reliability.
In regard to the wavelength region of from 0.85 .mu.m to 1.1 .mu.m, a development has been recently made on strained lattice lasers comprised of an active layer formed of In.sub.y Ga.sub.1-x As, which has enabled oscillation in this wavelength region. In the case of a heterostructure device, which is one of the strained lattice lasers, the device is required to have a lattice mismatch of not more than 10.sup.-3 to a substrate. In particular, in the case when it is constituted as a semiconductor laser, a defect may be introduced in its interior or no good heterointerface may be obtained if it has a large lattice mismatch, like the case of the semiconductor lasers of an InGaAs type. Hence, its crystals may undergo deterioration because of the heat generated when the device is operated, bringing about the problem in the reliability as a laser. The semiconductor lasers of his type can only have lifetimes and threshold values that are on the same level as those of the conventional semiconductor lasers of the AlGaAs type.
Stated additionally, the AlGaAs semiconductor lasers can attain an oscillation wavelength of about 0.7 .mu.m. However, because of an increase in the compositional proportion of aluminum in the composition of the active layer, a problem arises such that the active layer undergoes oxidation in the course of operation, resulting in a very short lifetime. Namely, the increase in the aluminum component in the active layer brings about the problems that the transition probability decreases, the light absorbance increases and the n-type doping becomes difficult because of a DX center. This makes the oscillation itself difficult to operate, and hence it has been difficult to put into practical use any semiconductor lasers having oscillation wavelengths of 0.78 .mu.m or less.
In particular, the AlGaInP semiconductor lasers (semiconductor light-emitting devices) can obtain laser light with the wavelength region of from 0.58 .mu.m to 0.67 .mu.m in theory. In such semiconductor lasers, band gaps become larger with an increase in the aluminum component in the active layer to tend to result in short oscillation wavelengths. Thus, as well known, an active layer of quantum well structure is used in order to improve laser characteristics, e.g., to make shorter the wavelength of oscillating laser light and to improve characteristic temperature (T.sub.0).
To more specifically discuss the active layer of quantum well structure, FIG. 5 illustrates the construction of a conventional AlGaInP semiconductor laser comprising an active layer having multiple quantum well (MQW) structure of an oxide film stripe type. In the drawing, the numeral 201 denotes a p-type electrode; 202, an insulating layer; 203, a p-GaAs ohmic contact layer; 204, a p-AlGaIn cladding layer; 205, an active layer; 206, an n-AlGaIn cladding layer; 207, a GaAs substrate; and 208, an n-type electrode. As shown in FIGS. 6A and 6B as partial enlarged views, the active layer 305 of the semiconductor laser is comprised of a well layer 308 made to have the quantum well structure and a barrier layer 309. These layers have, for example, the following composition and thickness.
Well layer: PA0 Barrier layer:
Composition: Ga.sub.0.5 In.sub.0.5 P PA1 Thickness: 10 nm PA1 Composition: (Al.sub.0.5 Ga.sub.0.5).sub.0.5 In.sub.0.5 P PA1 Thickness: 5 nm
Incidentally, such a semiconductor laser can obtain a laser beam with an oscillation wavelength of 0.655 .mu.m. The oscillation wavelength of this laser beam, however, becomes shorter by about 15 nm compared with a case when the active layer is merely comprised of Ga.sub.0.5 In.sub.0.5 P.
On the other hand, in this conventional semiconductor laser, an increase in the aluminum component in the active layer brings about an increase in oscillation threshold values or tends to cause oxidation of the active layer because of a high temperature at the time of operation, resulting in deterioration of laser characteristics and a short lifetime, as previously pointed out. In other words, in this conventional semiconductor laser discussed in the above, aluminum enters into the barrier layer when the active layer takes the quantum well structure, so that the aluminum in the barrier layer undergoes oxidation because of the high temperature at the time of operation to cause deterioration of the various characteristics previously noted.